1. Field of the Invention
The present invention relates generally to a door locking and unlocking system for an automotive vehicle, which includes a radio signal transmitter generating a self-identifying radio code signal for operating the door lock. More specifically, the invention relates to a door locking and unlocking system which improves system response with wider variations of radio transmitter identifying codes.
2. Description of the Prior Art
Automotive keyless entry systems have been disclosed in the European Patent First Publication Nos. 01 38 090, 01 40 137 and 01 54 306, for example. Generally such keyless entry system comprises a controller mounted on a vehicle and a pocket portable radio signal transmitter. The controller is actuated in response to a push button to output a radio signal for triggering the transmitter. The transmitter is responsive to the radio signal from the controller to transmit a radio signal containing a preset unique code. The controller receive the radio signal from the transmitter to same with a preset code therein. The controller outputs a control signal for operating a vehicle devices, such as an automatic door lock, when the received code matches the preset code.
For the purpose of theft prevention, the unique codes set in radio signal transmitters have to be differentiated from those of other transmitters. As may be appreciated, the number of variations of the unique code is determined according to the number of digits or bits to be used for constituting the unique code. However, increasing of the number of digits or bits requires greater consumption of the electricity. Therefore, in considering the life of a battery used in the radio signal transmitter, it is desirable to limit the number of digits or bits for the unique code.
On the other hand, in case that the keyless entry system is triggered with a single trigger code for triggering the portable transmitters, a plurality of transmitters tend to respond to the trigger signal at the same timing to cause interference of respectively transmitted transmitter identifying codes containing radio signals to thereby cause malfunction in the control unit. From this point of view, it would be better to selectively trigger the portable transmitters so that the generated transmitter identifying codes containing radio signals will not interfer with each other.